CL 54410 and CL 56250 recently added use of the MOVOstore
instruction to improve performance.

However, we can't use the MOVOstore instruction on Plan 9,
because floating point operations are not allowed in the
note handler.

This change adds a configuration flag useSSE to enable the
use of SSE instructions for non-floating point operations.
This flag is enabled by default and disabled on Plan 9.
When this flag is disabled, the MOVOstore instruction is
not used and the MOVQstoreconst instruction is used instead.

Fixes #21599

Change-Id: Ie609e5d9b82ec0092ae874bab4ce01caa5bc8fb8
Reviewed-on: https://go-review.googlesource.com/58850Reviewed-by: Keith Randall <khr@golang.org>